The primary structure of factor D of the human alternative complement pathway will be determined and compared with other serine proteases. The N-terminal 53 amino acids have been determined; the active site histidine of factor D is contained within this region of the molecule. N-terminal sequences of the three cyanogen bromide peptides have been determined. The active site serine is present at position number 14 of the carboxyl terminal peptide. Factor D has been shown to specifically inhibit thrombin-induced platelet aggregation via competitive inhibition of thrombin binding to the platelet surface. Factor D binding to platelets has been characterized. In addition platelets have been shown to contain and secrete factor D and the alternative complement pathway control protein, beta 1H globulin.